Up to now, it was conventional to carry out pneumatic yarn threading operations through two-for-one yarn twisting spindle assemblies only when utilizing balloon limitors with such spindle assemblies. This was based on the fact that it was considered necessary to utilize a balloon limitor to form a gap between the spindle assembly and the balloon limitor for defining a path of travel of the air flow and yarn upwardly along the outside of the spindle assembly to prevent the flow of air and thus the yarn being pneumatically threaded from moving over into and interfering with the operation of adjacent two-for-one yarn twisting spindle assemblies. This arrangement is clearly illustrated in U.S. Pat. No. 3,975,893, issued Aug. 24, 1976, and assigned to the assignee of the present application.
As far as known by applicant, the only two-for-one yarn twisting spindle assemblies which have eliminated the use of balloon limitors were those spindle assemblies in which pneumatic threading of the yarn was not intended, such as described in GB-PS No. 681562.
Other prior art directed to compressed air operated threading devices may be seen in DE-PS No. 504249 dated 1929/30, and FR-PS No. 1045449, dated 1951/53. Pneumatic threading devices which are associated with two-for-one yarn twisting spindles are shown and described in DE-PS No. 12 89 470 of 1969 and U.S. Pat. No. 3,731,478, issued May 8, 1973.
Despite the general efforts for automation which have been intensified especially during the last decade, this prior art does not contain any suggestion which, in the sense of the present invention, could be applied to two-for-one yarn twisting spindle assembly having no balloon limitor. Obviously, the reason can only be seen in the fact that there was considerable prejudice among the experts to recognize or to face up with the problem underlying the present invention.